Contact for silicon heterojunction solar cells

ABSTRACT

A photovoltaic device and method include a substrate coupled to an emitter side structure on a first side of the substrate and a back side structure on a side opposite the first side of the substrate. The emitter side structure or the back side structure include layers alternating between wide band gap layers and narrow band gap layers to provide a multilayer contact with an effectively increased band offset with the substrate and/or an effectively higher doping level over a single material contact. An emitter contact is coupled to the emitter side structure on a light collecting end portion of the device. A back contact is coupled to the back side structure opposite the light collecting end portion.

RELATED APPLICATION DATA

This application is a Divisional application of co-pending U.S. patentapplication Ser. No. 13/604,198 filed on Sep. 5, 2012, which is aContinuation application of co-pending U.S. patent application Ser. No.13/163,137 filed on Jun. 17, 2011, incorporated herein by reference intheir entirety.

BACKGROUND

1. Technical Field

The present invention relates to photovoltaic devices, and moreparticularly to contact structures which improve performance ofheterojunction cells.

2. Description of the Related Art

Heterojunction with intrinsic thin layer (HIT) solar cells have improvedin efficiency (e.g., 23% efficiency in the laboratory and 21% efficiencyin production). The HIT cells are comprised of intrinsic/dopedhydrogenated amorphous silicon (a-Si:H) serving as front (emitter) andback contacts, on a crystalline silicon (c-Si) absorber with p-type orn-type doping. An advantage of HIT cells is the low depositiontemperature of a-Si:H (˜200° C.) which offers a lower thermal budget ascompared to conventional c-Si cell processes (˜1000° C.). The lowprocess temperature also permits the use of low-cost Si wafers bypreserving carrier lifetime.

Referring to FIG. 1, an energy band diagram of a conventional HIT cell10 on a p-type crystalline silicon (c-Si) substrate 12 is shown inequilibrium. The equilibrium Fermi level is denoted by E_(F), and theconduction band and valence band edges are denoted by E_(c) and E_(v),respectively. Open circuit voltage of the cell 10 is the difference ofthe quasi Fermi level for electrons at an emitter 14 (the equilibriumFermi level in n⁺ a-Si:H layer 16) and the quasi Fermi level for holesat a back contact 18 (the equilibrium Fermi level in p⁺ a-Si:H layer20). The emitter 14 includes a front contact 15. Intrinsic layer 22 isdisposed between layer 16 and substrate 12.

The band offsets and equilibrium Fermi level parameters are thefundamental material properties of a-Si:H and may only vary marginallyby changing the growth conditions. For high-quality a-Si:H, the measuredvalues of ΔE_(c) and ΔE_(c) are in the range of 0.1-0.2 eV and 0.4-0.5eV, respectively. The equilibrium Fermi level cannot move closer to theconduction band than 0.15-0.2 eV in n⁺ a-Si:H 16, and cannot move closerthan 0.4-0.45 eV to the valence band in p⁺ a-Si:H 20 by increasing thedoping concentration. This is because doping incorporation increases thedefect density in a-Si:H and eventually pins the Fermi level position.

Replacing a-Si:H with other compounds results in creating largerband-offsets. Such band-offsets may improve the open circuit voltage,but at the cost of reducing fill factor (FF). This is because thetunneling rate of majority carriers is lower through the largerband-offsets. In addition, the issue of low doping efficiency applies tothese compounds as well.

SUMMARY

A photovoltaic device and method include a substrate coupled to anemitter side structure on a first side of the substrate and a back sidestructure on a side opposite the first side of the substrate. Theemitter side structure or the back side structure include layersalternating between wide band gap layers and narrow band gap layers toprovide a multilayer contact with an effectively increased band offsetwith the substrate and/or an effectively higher doping level over asingle material contact. An emitter contact is coupled to the emitterside structure on a light collecting end portion of the device. A backcontact is coupled to the back side structure opposite the lightcollecting portion.

Another photovoltaic device includes a substrate coupled to an emitterside superlattice on a first side of the substrate and a back sidesuperlattice on a side opposite the first side of the substrate. Theemitter side superlattice and the back side superlattice both have aplurality of layers alternately including wide band gap layers andnarrow band gap layers to provide a multilayer contact with aneffectively increased band offset with the substrate, and/or effectivelyincreased doping level over a single material contact. An emittercontact is coupled to the emitter side superlattice on a lightcollecting end portion of the device. A back contact is coupled to theback side superlattice opposite the light collecting portion.

A method for fabricating a photovoltaic device includes providing asubstrate; forming a plurality of layers including alternating wide bandgap and narrow band gap layers to form a superlattice to provide amultilayer with an effectively increased band offset with the substrateand/or effectively increased doping level over a single materialcontact; forming a conductive contact on the superlattice; and forming acontact structure on the substrate on a side opposite the superlattice.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is an energy band diagram of a conventional heterojunction withintrinsic thin layer (HIT) cell on a p-type crystalline silicon (c-Si)substrate;

FIG. 2 is a cross-sectional view of a heterojunction solar cell withsuperlattices on front and back contacts (sides) on a p-type c-Sisubstrate in accordance with one embodiment;

FIG. 3 is a cross-sectional view of a heterojunction solar cell withsuperlattices on front and back contacts (sides) on an n-type c-Sisubstrate in accordance with another embodiment;

FIG. 4 is an energy band diagram of a heterojunction solar cell on ap-type crystalline silicon (c-Si) substrate witha-Si_(x)C_(1-x):H/a-Si_(y)Ge_(1-y):H front anda-Si_(x)Ni_(1-x):H/a-Si_(y)Ge_(1-y):H back superlattice contacts inaccordance with one embodiment;

FIG. 5 is a cross-sectional view of a solar cell device with aconventional heterojunction back contact and a conventional diffusedemitter (front) contact;

FIG. 6 is a cross-sectional view of a solar cell device with asuperlattice heterojunction back contact and a conventional diffusedemitter contact in accordance with one embodiment; and

FIG. 7 is a block/flow diagram of a method for fabricating theheterojunction solar cell device in accordance with the presentprinciples.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the present principles, superlattice contactstructures and methods for forming the same are provided forheterojunction solar cells. The superlattice structure improves opencircuit voltage of the cell without compromising fill factor (FF). Theopen-circuit voltage is improved due to a larger effective band-offsetof the superlattice contact and/or modulation (transfer) doping of a lowband gap material in the superlattice without pinning the Fermi level.For example, n-type doping may be enhanced by the transfer of electronsfrom the wide bandgap material to the narrow band gap material or thetransfer of holes from the narrow bandgap material to the wide band gapmaterial. Likewise, p-type doping may be enhanced by the transfer ofholes from the wide bandgap material to the narrow band gap material orthe transfer of electrons from the narrow bandgap material to the wideband gap material. The fill factor is not compromised by overall reducedthickness of wide band gap material and/or the creation of mini-bandsdue to quantum confinement in narrow band gap material. The narrow bandgap material and the mini-band creation enhance the majority carriertunneling through the superlattice.

It is to be understood that the present invention will be described interms of a given illustrative architecture having a wafer or substrate;however, other architectures, structures, substrate materials andprocess features and steps may be varied within the scope of the presentinvention.

It will also be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

Methods as described herein may be used in the fabrication ofphotovoltaic cells and chips or modules including the same. Theresulting integrated chips or devices can be distributed by thefabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In anycase, the chip or device may be integrated with other chips, discretecircuit elements, and/or other signal processing devices as part ofeither (a) an intermediate product, such as a motherboard, or (b) an endproduct. The end product can be any product that includes integratedcircuit chips, ranging from toys and other low-end applications toadvanced computer products having a display, a keyboard or other inputdevice, and a central processor.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 2, an illustrative structureof a photovoltaic device 100 with a p-type substrate 102 is shown. Thesubstrate 102 preferably includes silicon and may be single-crystalline(c-Si) or microcrystalline (μc-Si). The substrate 102 includes anemitter or front contact and a back contact. The word “contact” isemployed to refer to the structure connected to the substrate 102, whichmay include an emitter/front contact or a back contact. Since thesecontacts also include conductive contacts as well, to prevent confusion,the contacts will be referred to as emitter side 110 and back side 130.

In accordance with the present principles, an emitter side 110 of thesubstrate 102 includes a superlattice stack of layers 120. The stack 120of layers includes alternating wide band gap (high band gap or wide gap)layers 122 (e.g., doped layers) and narrow band gap (low band gap ornarrow gap) layers 124 (e.g., intrinsic layers). The number of layers nis greater than 2. Wide gap layers 122 include a wide band gapsemiconductor, which is a semiconductor material with an electronic bandgap larger than one or two electronvolts (eV). The narrow gap layers 124include semiconducting materials with a band gap that is comparativelysmall compared to silicon. Each wide-gap layer and narrow-gap layer mayinclude a doped layer (n⁺ dopants) or an undoped layer, or an intrinsiclayer (i-layer). An optional intrinsic layer 126 may be interposedbetween the stack 120 and the substrate 102. In one embodiment, thesuperlattices 120, 132 include alternating doped and intrinsic layers.

Semiconducting material(s) forming passivation intrinsic layers ori-layer(s) may include a-Si:H, a-Ge:H, a-SiGe_(x):H, a-SiN_(x):H,a-SiO_(x):H, a-SiC_(x):H, or combinations of these materials. Thesemiconducting material(s) forming doped/undoped layer(s) may includeamorphous, nanocrystalline, microcrystalline or polycrystalline films(s)of Si, Ge, SiGe_(x), SiC_(x), SiO_(x), SiN_(x), or combinations of thesematerials and may or may not contain hydrogen. The films forming thedoped or undoped layers may or may not contain fluorine or deuterium.The doped layers may include appropriate n-type or p-type dopantmaterials. Examples of the high-bandgap/low-bandgap materialcombinations for the superlattice 120 includea-Si_(x)C_(1-x):H/a-Si_(y)Ge_(1-y):H,a-Si_(x)Ni_(1-x):H/a-Si_(y)Ge_(1-y):H,a-Si_(x)O_(1-x):H/a-Si_(y)Ge_(1-y):H, where 0≦x<<1 and 0≦y≦1. Thethickness of the layers 122, 124 is preferably less than 20 nm and morepreferably in the range of 1 nm-20 nm with the overall thickness of theemitter side 110 preferably being below ˜15 nm (to reduce lightabsorption).

It should be understood that the present embodiments preferably employdoping techniques that inflict minimal damage to the doped layers.Doping of semiconductors is the process of locally manipulating theircharge carrier density and conductivity. Conventional doping is usuallyachieved via the bombardment of semiconductors with dopants followed bythermal annealing (ion implantation). Bombardment of materials withenergetic ions can create severe crystal damage, in particular, fornanoscale layers, thereby degrading the device performance.

In accordance with the present principles, a nondestructive dopingmethod is preferably employed to dope the materials of thesuperlattice(s) 120 and 132. This employs charge exchange between twomaterials (e.g., a semiconductor material and a selected dopant materialor doped material and an intrinsic material, etc.) at their interface,and is referred to as modulated or transfer doping. The process employsmisalignment of the Fermi levels on each side to utilize the interfacialcharge transfer as an effective doping scheme to control theconductivity of nano-structured materials. By selecting the stackingmaterials in the superlattice structures 120 and 132, transfer doping isemployed to increase effective doping and conductivity (e.g., overconventional a-Si doped layers) since lattice damage is avoided.

The back side 130 of the substrate 102 includes a superlattice stack oflayers 132. The stack of layers 132 includes alternating wide gap (highband gap) layers 142 and narrow gap (low band gap) layers 144. Thenumber of layers m is greater than 2. Each wide gap layer and narrow gaplayer may include a doped layer (p⁺ dopants) or an undoped layer, or anintrinsic layer (i-layer). An optional intrinsic layer 146 may beinterposed between the stack 132 and the substrate 102.

Semiconducting material(s) forming passivation intrinsic layers ori-layer(s) may include a-Si:H, a-Ge:H, a-SiGe_(x):H, a-SiN_(x):H,a-SiO_(x):H, a-SiC_(x):H, or combinations of these materials. Thesemiconducting material(s) forming doped/undoped layer(s) may includeamorphous, nanocrystalline, microcrystalline or polycrystalline films(s)of Si, Ge, SiGe_(x), SiC_(x), SiO_(x), SiN_(x), or combinations of thesematerials and may or may not contain hydrogen. The films forming thedoped or undoped layers may or may not contain fluorine or deuterium.The doped layers may include appropriate n-type or p-type materials.Examples of the high band gap/low band gap material combinations for thesuperlattice 132 include a-Si_(x)C_(1-x):H/a-Si_(y)Ge_(1-y):H,a-Si_(x)Ni_(1-x):H/a-Si_(y)Ge_(1-y):H,a-Si_(x)O_(1-x):H/a-Si_(y)Ge_(1-y):H, where 0≦x<<1 and 0≦y≦1. Thethickness of the layers 142, 144 is preferably less about 20 nm and morepreferably in the range of 1 nm-20 nm with the overall thickness of theback side 130 being below ˜50 nm.

An emitter top portion 150 may include a conductive layer 152 and anoptional contact layer (n⁺) 154. The contact layer 154 may connect withthe emitter side 110 stack 120. The contact layer 154 may includeamorphous, nanocrystalline, microcrystalline or polycrystalline films(s)of Si, Ge, SiGe_(x), SiC_(x), SiO_(x), SiN_(x), or combinations of thesematerials and may or may not contain hydrogen and may or may not containfluorine or deuterium. The conductive layer 152 may include a metal suchas Aluminum, Silver, Tungsten, etc. and/or conductive layer 152 mayinclude a transparent conductive oxide, such as indium tin oxide, zincoxide, etc. Other structures may also be included such as conductivefingers 155, anti-reflection coatings, protective coatings, etc.

A back portion 160 may include a conductive layer 162 and an optionalcontact layer (pt) 164. The contact layer 164 may connect with the backside 130 stack 132. The contact layer 164 may include amorphous,nanocrystalline, microcrystalline or polycrystalline films(s) of Si, Ge,SiGe_(x), SiC_(x), SiO_(x), SiN_(x), or combinations of these materialsand may or may not contain hydrogen and may or may not contain fluorineor deuterium. The conductive layer 162 may include a metal such asAluminum, Silver, Tungsten, etc., and/or conductive layer 162 mayinclude a transparent conductive oxide, such as Indium Tin Oxide, ZincOxide, etc. Other structures may be included as well. For example,antireflection coatings may be formed or conductive fingers 155, 170 maybe employed.

At least one of layers 152 and 162 is composed of a transparentconductive material to permit light to enter the absorption layer 102.If both layers 152 and 162 are composed of transparent conductivematerials, the light can enter from both sides of the cell, and the cellis referred to as a bifacial cell. If layer 152 (or 162) is composed ofa transparent conductive material, metal fingers 155 (or 170) are neededto allow for low electrical contact resistance, while if layer 152 (or162) is composed of a metal, the electrical conductivity of the contactis sufficient and the metal fingers 155 (or 170) are not needed. Themetal used for layer 152 or 162 may have reflective properties.

Referring to FIG. 3, an illustrative structure of another photovoltaicdevice 200 with an n-type substrate 202 is shown. The substrate 202preferably includes silicon and may be single-crystalline (c-Si) ormicrocrystalline (μc-Si). In accordance with the present principles, anemitter side 210 of the substrate 202 includes a superlattice stack 220of layers. The stack 220 of layers includes alternating wide gap (highband gap) layers 222 and narrow gap (low band gap) layers 224. Thenumber of layers n>2. Wide-gap layers 222 include a wide band gapsemiconductor, which is a semiconductor material with an electronic bandgap larger than one or two electronvolts (eV). The narrow gap layers 224include semiconducting materials with a band gap that is comparativelysmall compared to silicon. Each wide gap layer and narrow gap layer mayinclude a doped layer (p⁺ dopants), an undoped layer or an intrinsiclayer (i-layer). An optional intrinsic layer 226 may be interposedbetween the stack 220 and the substrate 202. In one embodiment, thesuperlattices 220, 232 include alternating doped and intrinsic layers.

Semiconducting material(s) forming passivation intrinsic layers ori-layer(s) may include a-Si:H, a-Ge:H, a-SiGe_(x):H, a-SiN_(x):H,a-SiO_(x):H, a-SiC_(x):H, or combinations of these materials. Thesemiconducting material(s) forming doped/undoped layer(s) may includeamorphous, nanocrystalline, microcrystalline or polycrystalline films(s)of Si, Ge, SiGe_(x), SiC_(x), SiO_(x), SiN_(x), or combinations of thesematerials and may or may not contain hydrogen. The films forming thedoped or undoped layers may or may not contain fluorine or deuterium.Examples of the high band gap/low band gap material combinations for thesuperlattice include a-Si_(x)C_(1-x):H/a-Si_(y)Ge_(1-y):H,a-Si_(x)Ni_(1-x):H/a-Si_(y)Ge_(1-y):H,a-Si_(x)O_(1-x):H/a-Si_(y)Ge_(1-y):H, where 0≦x<<1 and 0≦y≦1. Thethickness of the layers 222, 224 is preferably less than 20 nm and morepreferably in the range of 1 nm-20 nm with the overall thickness of theemitter side 210 preferably being below ˜15 nm (to reduce lightabsorption in the contact). Doping is preferably performed using anon-destructive technique (e.g., doping during layer growth, modulatedor transfer doping, etc.).

The back side 230 of the substrate 202 includes a superlattice stack 232of layers. The stack of layers 232 includes alternating wide gap (highband gap) layers 242 and narrow gap (low band gap) layers 244. Thenumber of layers m>2. Each wide gap layer and narrow gap layer mayinclude a doped layer (n⁺ dopants) or an undoped layer, or an intrinsiclayer (i-layer). An optional intrinsic layer 246 may be interposedbetween the stack 232 and the substrate 202.

Semiconducting material(s) forming passivation intrinsic layers ori-layer(s) may include a-Si:H, a-Ge:H, a-SiGe_(x):H, a-SiN_(x):H,a-SiO_(x):H, a-SiC_(x):H, or combinations of these materials. Thesemiconducting material(s) forming doped/undoped layer(s) may includeamorphous, nanocrystalline, microcrystalline or polycrystalline films(s)of Si, Ge, SiGe_(x), SiC_(x), SiO_(x), SiN_(x), or combinations of thesematerials and may or may not contain hydrogen. The films forming thedoped or undoped layers may or may not contain fluorine or deuterium.Examples of the high band gap/low band gap material combinations for thesuperlattice include a-Si_(x)C_(1-x):H/a-Si_(y)Ge_(1-y):H,a-Si_(x)Ni_(1-x):H/a-Si_(y)Ge_(1-y):H,a-Si_(x)O_(1-x):H/a-Si_(y)Ge_(1-y):H, where 0≦x<<1 and 0≦y≦1. Thethickness of the layers 242, 244 is preferably in the range of 1 nm-20nm with the overall thickness of the back side 230 being below ˜50 nm.

An emitter top portion 250 may include a conductive layer 252 and anoptional doped contact layer (p⁺) 254. The contact layer 254 may connectwith the emitter side 210 stack 220. The contact layer 254 may includeamorphous, nanocrystalline, microcrystalline or polycrystalline films(s)of Si, Ge, SiGe_(x), SiC_(x), SiO_(x), SiN_(x), or combinations of thesematerials and may or may not contain hydrogen and may or may not containfluorine or deuterium. The conductive layer 252 may include a metal suchas Aluminum, Silver, Tungsten, etc., and/or conductive layer 252 mayinclude a transparent conductive oxide, such as Indium Tin Oxide, ZincOxide, etc.

A back portion 260 may include a conductive layer 262 and an optionalcontact layer (n⁺) 264. The contact layer 264 may connect with the backside 230 stack 232. The contact layer 264 may include amorphous,nanocrystalline, microcrystalline or polycrystalline films(s) of Si, Ge,SiGe_(x), SiC_(x), SiO_(x), SiN_(x), or combinations of these materialsand may or may not contain hydrogen and may or may not contain fluorineor deuterium. The conductive layer 262 may include a metal such asAluminum, Silver, Tungsten, etc., and/or conductive layer 162 mayinclude a transparent conductive oxide, such as Indium Tin Oxide, ZincOxide, etc. Other structures may be included as well. For example,antireflection coatings may be formed or conductive fingers 270 may beemployed. At least one of the layers 252 and 262 is composed of atransparent conductive material to allow the light to enter theabsorption layer 202. If both layers 252 and 262 are composed oftransparent conductive materials, the light can enter from both sides ofthe cell (a bifacial cell). If layer 252 (or 262) is composed of atransparent conductive material, metal fingers 270 are needed on thatrespective side to allow for low electrical contact resistance, while iflayer 252 (or 262) is composed of a metal, the electrical conductivityof the contact is sufficient and the metal fingers 255 (or 270) are notneeded. The metal type used for layer 252 or 262 is preferablyreflective.

Referring to FIG. 4, a schematic energy band diagram of a p-type cell300 with superlattice contacts (304, 312) coupled to a p-type silicon(c-Si) substrate 301 is illustratively shown. An emitter or frontcontact 302 is a conductive material. An emitter side stack 304 includesa plurality of alternating layers 306, 308 as described with respect toFIG. 2. In this example, the layers 306 include a-Si_(x)C_(1-x):H, andlayers 308 include a-Si_(y)Ge_(1-y):H. A back contact 310 includes aconductive material. A back side stack 312 includes a plurality ofalternating layers 314, 316 as described with respect to FIG. 2. In thisexample, the layers 314 include a-Si_(x)Ni_(1-x):H, and layers 316include a-Si_(y)Ge_(1-y):H. Optional intrinsic layers 318 are includedin this example.

As a result of the superlattice stacks 304 and 312, open circuit voltageis increased, without compromising the fill factor. To increase the opencircuit voltage, the conduction band offset (ΔE_(c)) is increased in theemitter 304 and/or the valence band offset (ΔE_(v)) is increased in theback 312, and/or the doping efficiency of the stacked layers 306/308 and314/316 is effectively increased to raise the quasi Fermi level positionat the emitter 304 and/or increase the doping efficiency to lower thequasi Fermi level position in the back 312.

At least one of the band offsets is preferably increased. For example,the conduction band offset (ΔE_(c)) is increased on the emitter sidestack 304, and/or the valence band offset (ΔE_(v)) is increased on theback side stack 312. This increases the separation of the quasi-Fermilevels for electrons and holes, and as a result, improves the opencircuit voltage. The enhanced tunneling through the superlatticecontacts permits a high fill factor which could otherwise be compromisedif the increased band-offset were created by a single semiconductorcontact material. The enhanced tunneling is due to the creation ofmini-bands as a result of quantum confinement in the superlattice,and/or due to the enhanced electric field resulting from the improveddoping of the narrow gap material by transfer (modulation) doping of thenarrow gap material. The improved doping also further improves theopen-circuit voltage of the cell by improving the electric field at thefront and/or back junctions further increasing the separation of thequasi-Fermi levels.

Unlike conventional substitutional doping which requires theincorporation of impurity atoms in the lattice of an amorphous materialresulting in the creation of extra defects and the degradation of dopingefficiency, transfer (modulation) doping in accordance with the presentprinciples does not involve the incorporation of impurity atoms, and asa result, the issue of Fermi level pinning is substantially suppressed.The equilibrium Fermi level can thus move closer to the conduction bandon the n⁺ contact side and closer to the valence band on the p⁺ contactside.

Referring to FIG. 5, a solar cell device 400 with a conventionaldiffused emitter (front) contact, and a conventional heterojunction backcontact is shown. The cell 400 includes a metal back contact 402 and ap⁺ a-Si:H (15 nm) back side doped layer 404. An intrinsic layer (5 nm)406 is disposed between layer 404 and a p-type substrate 405 (e.g.,c-Si). An emitter side includes an n⁺ diffusion layer 408 and conductivefingers 410. An antireflection coating (ARC) 412 is also employed. Inoperation, this structure 400 provided an open circuit voltage (V_(oc))of about 640 mV and a fill factor of about 72%. Switching the layer 404to p⁺ a-Ge:H of the same thickness reduced the V_(oc) to about 580 mVand provided a fill factor (FF) of about 76%.

Referring to FIG. 6, a heterojunction cell 500 is shown in accordancewith the present principles. The cell 500 includes back contact 402 butincludes an alternating stack 504 for the back side. The stack 504includes p⁺ a-Si:H (5 nm) layers 510 which sandwich a layer 512 ofa-Ge:H (5 nm). In operation, this structure 500 provides a V_(oc) ofabout 665 mV and a fill factor (FF) of about 80%. Other improvements ofV_(oc) and FF may also be obtained by material selection and structuresin accordance with the present principles.

The layers of stack 504 provide a combination of lower tunneling barrierand higher doping efficiency, which results in higher V_(oc) and fillfactor. It should be understood that the superlattice structures inaccordance with the present principles may be provided for both frontand back contacts. However, the devices using one superlattice on oneside of the substrate and a conventional contact structure on the otherside are also within the scope of the present principles (see, e.g.,FIG. 6).

Referring to FIG. 7, a method for fabricating a photovoltaic device isillustratively shown. The flowchart/block diagram of FIG. 7 illustratesthe architecture, functionality, and operation of possibleimplementations according to various embodiments of the presentinvention. In this regard, the functions noted in the block diagram mayoccur out of the order noted in the figure. For example, two blocksshown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved.

In block 602, a substrate or wafer is provided. The substrate mayinclude a p-type or n-type substrate. A p-type substrate may includen-type doped layers on an emitter side and/or p-type doped layers on aback side. An n-type substrate may include p-type doped layers on anemitter side and/or n-type doped layers on a back side. The substratepreferably includes a single crystal or crystalline silicon (c-Si),although other substrate materials may be employed. In block 604, anoptional intrinsic layer may be formed on one or both sides of thesubstrate.

In block 606, a superlattice is formed on at least one side of thesubstrate (or on an intrinsic layer if present). The superlatticeincludes a plurality of layers alternating between wide band gap andnarrow band gap materials. The electrical effect of the superlattice isequivalent to that of a single layer semiconductor material with ahigher conduction or valence band offset with the substrate as comparedto between a-Si:H and the substrate, and/or is equivalent to a higherdoping efficiency compared to that of a-Si:H. For example, on theemitter side, a conduction band offset may be effectively increased, andon the back side a valence band offset may be effectively increased. Thesuperlattice layers may be formed using different techniques that dependon the type of materials and the desired properties. In usefulembodiments, chemical vapor deposition (CVD), plasma enhanced CVD(PECVD) or other methods may be employed. The layers preferably includea thickness of less than about 20 nm to increase doping efficiency forthose layers that include dopants. Higher tunneling current is achievedin this way due to quantum confinement by forming mini-bands(sub-bands), and/or due to electric field enhancement at the junctionsresulting from improved doping efficiency with the plurality of layersof the superlattice.

In one example, for a crystalline silicon substrate, the wide band gaplayers may include one of amorphous silicon carbide and amorphoussilicon nitride and the narrow band gap layers may include one ofamorphous silicon germanium and nano- or micro crystalline silicon.Higher conductivity is achieved by leveraging transfer doping betweenadjacent layers in the plurality of layers of the superlattice.

In block 608, a doped contact layer may optionally be formed between thesuperlattice and conductive contact. In block 610, a conductive contactis formed on the superlattice (or optional doped contact layer). Theconductive contact may include a metal or transparent conductivematerial such as a transparent conductive oxide.

In block 612, a contact structure is formed on the substrate on a sideopposite the superlattice. This may include a conventional contactstructure or another superlattice. The superlattice formed includes thesame structure as described above with respect to block 606 except anydoped layer would include an opposite polarity with respect to thesuperlattice on the opposite side of the substrate. A conductive contactis also formed on the end of the superlattice. Also, an optionalintrinsic layer and doped contact layer may optionally be formed. Inblock 614, continued processing occurs. This may include the formationof antireflection coatings, conductive fingers, protective layers, etc.

Having described preferred embodiments for improved contact for siliconheterojunction solar cells (which are intended to be illustrative andnot limiting), it is noted that modifications and variations can be madeby persons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A photovoltaic device, comprising: a substratecoupled to an emitter side structure on a first side of the substrateand a back side structure on a side opposite the first side of thesubstrate; an emitter side structure comprising an n⁺ diffusion layer; aback side structure engaged to the emitter side structure, the back sidestructure including a plurality of layers alternately including wideband gap layers and narrow band gap layers to provide a multilayercontact with at least one of an effectively increased band offset withthe substrate and/or an effectively higher doping level over a singlematerial contact; an emitter contact coupled to the emitter sidestructure on a light collecting end portion of the device; and a backcontact coupled to the back side structure opposite the light collectingend portion.
 2. The photovoltaic device as recited in claim 1, whereinthe back side structure including doped layers and intrinsic layers. 3.The photovoltaic device as recited in claim 1, wherein the substrateincludes a p-type substrate or an n-type substrate.
 4. The photovoltaicdevice as recited in claim 1, wherein the substrate includes crystallinesilicon and the wide band gap layers include one of amorphous silicon,amorphous silicon carbide and amorphous silicon nitride.
 5. Thephotovoltaic device as recited in claim 1, wherein the substrateincludes crystalline silicon and the narrow band gap layers include oneof amorphous silicon, amorphous germanium, amorphous silicon germaniumand nano- or micro crystalline silicon.
 6. The photovoltaic device asrecited in claim 1, further comprising an intrinsic layer disposedbetween the substrate and the at least one of the emitter side structureand the back side structure.
 7. The photovoltaic device as recited inclaim 1, further comprising a contact layer disposed between the backcontact and the back side structure.
 8. The photovoltaic device asrecited in claim 1, wherein the wide band gap layers and the narrow bandgap layers each have a thickness of less than about 20 nm.
 9. Aphotovoltaic device, comprising: a substrate coupled to an emitter siden⁺ diffusion layer on a first side of the substrate and a back sidesuperlattice on a side opposite the first side of the substrate; theback side superlattice both having a plurality of layers alternatelyincluding wide band gap layers and narrow band gap layers to provide amultilayer contact with an effectively increased band offset with thesubstrate, and/or effectively increased doping level over a singlematerial contact; an emitter contact coupled to the emitter side n⁺diffusion layer on a light collecting end portion of the device; and aback contact coupled to the back side superlattice opposite the lightcollecting end portion.
 10. The photovoltaic device as recited in claim9, wherein the back side superlattice each include doped layers andintrinsic layers.
 11. The photovoltaic device as recited in claim 9,wherein the substrate includes a p-type substrate or an n-typesubstrate.
 12. The photovoltaic device as recited in claim 9, whereinthe substrate includes crystalline silicon and the wide band gap layersinclude one of amorphous silicon, amorphous silicon carbide andamorphous silicon nitride.
 13. The photovoltaic device as recited inclaim 9, wherein the substrate includes crystalline silicon and thenarrow band gap layers include one of amorphous silicon, amorphousgermanium, amorphous silicon germanium and nano- or micro crystallinesilicon.
 14. The photovoltaic device as recited in claim 9, furthercomprising an intrinsic layer disposed between the substrate and theback side superlattice.
 15. The photovoltaic device as recited in claim9, further comprising a contact layer disposed between the back contactand the back side superlattice.
 16. The photovoltaic device as recitedin claim 9, wherein the wide band gap layers and the narrow band gaplayers each have a thickness of less than about 20 nm.